Method for producing pearlescent basic lead carbonate

ABSTRACT

A pearlescent pigment consisting of hexagonal crystalline platelets of basic lead carbonate is produced by bubbling a diluted stream of carbon dioxide gas into an aqueous solution of a basic lead salt which is derived from the reaction of lead monoxide with an acid. The acid that is reacted with the lead monoxide can be nitric acid, but more preferably will have the general formula: WHEREIN X is a member from the group consisting of hydrogen, methyl, and halogen. The carbon dioxide stream that is bubbled into the salt solution can be diluted with any suitable gas that is substantially nonreactive with the basic lead salt, e.g. air, nitrogen, or one of the inert gases, thus providing control over the rate and extent of crystal initiation and growth. The resulting basic lead carbonate pigment can be incorporated into plastics, lacquers and the like to provide articles having the appearance of natural pearl.

United States Patent Eckert et al.

[ 1 Dec. 19,1972

I541 METHOD FOR PRODUCING PEARLESCENT BASIC LEAD CARBONATE [72] Inventors: Frank .I. Eckert, Morrisville, Pa.; David R. Meldrum, Englishtown, NJ.

[73] Assignee: Cities Service Company, New York,

[22] Filed: Oct. 22, 1970 [21] Appl. No.: 83,185

[52] US. Cl. ..l06/291, 106/297, 106/309 [51] Int. Cl ..C09c 1/14 [58] Field of Search ..l06/29l 297, 309; 23/71 [56] References Cited FOREIGN PATENTS OR APPLICATIONS 500,961 2/1939 Great Britain ..l06/297 Primary Examiner.lames E. Poer Assistant Examiner-J. V. Howard Attorney-J. Richard Geaman [57] ABSTRACT A pearlescent pigment consisting of hexagonal crystalline platelets of basic lead carbonate is produced by bubbling a diluted stream of carbon dioxide gas into an aqueous solution of a basic lead salt which is derived from the reaction of lead monoxide with an acid. The acid that is reacted with the lead monoxide can be nitric acid, but more preferably will have the general formula:

16 Claims, No Drawings METHOD FOR PRODUCING PEARLESCENT BASIC LEAD CARBONATE BACKGROUND or THE INVENTION The term pearl essence refers to a composition prepared from the minute guanine and hypoxanthane crystals which are extracted from the skin and scales of certain fish. This natural pearl essence can be incorporated into nitrocellulose or methyl methacrylate to provide artificial pearls or buttons, jewelery, decorative finishes, and the like which resemble real pearls or mother-of-pearl. Since the recovery of natural pearl essence is a lengthy and very costly process, it has in the large part been replaced by synthetically produced pigments such as basic lead carbonate and bismuth oxychloride. All such pigments, whether natural or synthetic, are referred to as pearlescent or nacreous pigments.

The most popular synthetic pearlescent pigment is basic lead carbonate which is precipitated by passing carbon dioxide gas into an aqueous solution of 'a basic lead salt which has been formed by reacting lead monoxide, or litharge, with an aqueous solution of an acid, and the term basic lead salt as used herein thus refers to a double salt of lead. For instance, reaction of litharge with an aqueous solution of nitric acid produces an aqueous solution of a basic lead salt having the formula:

Reaction of litharge with an aqueous solution of acetic acid produces an aqueous solution of a basic lead salt having the formula:

Corresponding double salts of lead are produced by reaction of litharge with still other acids. Basic lead salts, therefore, have the general formula:

where X is representative of the anion of the water soluble acid which is reacted with the lead monoxide.

Pearlescent basic lead carbonate crystals consist of 2PbCO -Pb(Ol-l) or a mixture of both. In precipitating the basic lead carbonate pigment, conditions are established and maintained in an attempt to provide hexagonal crystalline platelets that are characterized by clarity, or transparency, and platelet dimensions that are essential in providing a maximum pearlescent effect. l-leretofore, optimum platelet dimensions for a pearlescent basic lead carbonate pigment have been specified as being about 5 to about 50 microns in diameter and about 10 to about 50 millimicrons in thickness. It will be appreciated that these specifications apply to pigments which are a mixture of platelets having widely different diameters and thicknesses within these ranges, rather than being applicable to a number of grades of the pigment wherein the platelet dimensions are relatively uniform in each grade, but significantly different from one grade to the next.

Broad specifications on platelet size have been necessary heretofore since it has not been possible to finely control the rate and extent of the crystal growth of basic lead carbonate, and as a result the crystal platelets which were produced have been characterized by a broad range'of diameters and thicknesses.

SUMMARY OF THE INVENTION The present invention is based on the discovery that the reaction which occurs when carbon dioxide is bubbled into an aqueous solution of a basic lead salt, for production of pearlescent basic lead carbonate pigment, has heretofore been largely uncontrolled because of nonuniform distribution of the carbon dioxide throughout the aqueous reaction medium. Furthermore, the carbon dioxide gas has always been undiluted when bubbled into the salt solution, and in such a case the'reaction tends to proceed very rapidly in the region where the carbon dioxide first enters the salt solution, and at still other and somewhat slower rates in other regions of the solution. As a consequence of this nonuniform reaction rate throughout the solution, it has not been possible to produce pearlescent pigments of basic lead carbonate which are characterized by relatively uniform platelet dimension since an excess of undesirably small platelets are produced by the fast, localized reaction.

In accordance with the present invention, the carbon dioxide stream which is introduced into the basic lead salt solution is diluted with another gas which is substantially nonreactive with the basic lead salt. By means of this technique, the reaction between the carbon dioxide and the salt can be largely delocalized, thus permitting the reaction to proceed much more smoothly and uniformly throughout the solution. In addition, dilution of the carbon dioxide gas stream with an inert gas seems to influence crystal initiation and growth to the extent that the resulting platelets are more uniform in size while also having diameter and the thickness dimensions which are especially preferably. In one embodiment of the invention, the carbon dioxide can be passed through the solution as relatively large bubbles, e.g. in excess of one-quarter inch in diameter, thus providing a supplimentary means for delocalizing the reaction.

Basic lead carbonate pigments can be produced in accordance with this invention which can highly regular hexagonal crystal platelets having an average diameter within the range of about 10 to about 20 microns and an average thickness within the range of about 10 to about millimicrons. More preferably, the resulting pigment will have an average diameter within the range of about 12 to about 18 microns and a thickness within the range of about 10 to about 50 millimicrons. As will be appreciated, the brilliance and luster of basic lead carbonate pigments which can be produced in accordance with this invention are greatly enhanced by virtue of the fact that the pearlescent effect is not unduly hindered by an over abundance of crystal platelets which are too small and/or too large to provide a maximum pearlescent effect. A-very high quality synthetic pearl essence can thus be manufactured by practice of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Any suitable water-soluble basic lead salt can be employed in this invention for producing the pearlescent basic lead carbonate pigment. Nitric acid can be used to produce a suitable salt, but the preferred salts are produced by dissolving lead monoxide vin an aqueous solution of an organic acid having the general formula:

n x-cHr-aH-c-H X wherein X is a member of the group consisting of hydrogen, methyl and halogen. Organic acids that can be employed to advantage for making the basic lead salt include acetic 1 acid, propionic acid, chloropropionic acid, isobutyric acid,- and n-butyric acid. 7

Any suitable gas that is not substantially-nonreactive with the basic lead salt can be employed as a diluent for the carbon dioxide stream that is introduced into the aqueous solution of the salt. Air, nitrogen, or one of the inert gases can be employed for the purpose, but air can be used to particular advantage since it is so readily available. The extent to which the carbon dioxide should be diluted with the nonreactive gas is subject to considerable variation. At least 25 volume percent of the diluent gas can be mixed with the carbon dioxide to provide an advantage, but higher concentrations are usually more advantageous, e.g. from about 50 to about 80 volume percent of the diluent gas, and higher, can be employed. It will be apparent, of course, that the carbon dioxide stream should not be so highly diluted that the reaction period is thereby extended to an unacceptable length of time. On the other hand, the extent of reaction between carbon dioxide and a basic lead salt in accordance with the invention is not substantially reduced on an over-all basis. Therefore, the proportion of diluting gas to carbon dioxide which should be employed in any particular case can be readily determined by a few simple experiments guided by the good judgment of one skilled in the art.

In accordance with conventional practice, homogeneous distribution of the carbon dioxide throughout the solution of basic lead salt can be I bon dioxide with the basic lead salt can be rendered more uniform throughout the aqueous reaction medium if the carbon dioxide is passed into the medium as relatively large bubbles. In effect, this reduces the interfacial contact area between the gas and the liquid and thus reduces seeding and growth rates of the crystals. Therefore, in accordance with the present invention, the diluted stream of dioxide can, to further advantage, be passed through the aqueous solution of basic lead salt as bubbles having a diameter of atleast about one-fourth of an inch. More preferably, the bubbles can have a diameter within the range of about onehalf to about 1 inch. In accordance with the prior art, undiluted carbon dioxide has been passed into the solution as bubbles having a diameter-of about one-sixteenth-of an inch or less.

In the present process the lead content of the basic lead salt solution is preferably within the range of abut 0.5 to about 7.0 weight percent although solutions which are somewhat more or less concentrated can also be employed. When first made up, the basic lead salt solution may have a content of lead which is somewhat higher than preferred, but a more dilute solution can be obtained simply by cutting back with water to achieve the concentration desired.

Prior to initiating the reaction, the solution of basic lead salt maybe allowed to stand for several hours to allow small, solid particles .of objectionable matter to grow or coalesce to a larger size. These particles should then be removed from the solution by filtering or decanting prior tov introduction of the carbon dioxide since their'presence can deleteriously affect formation and growth of the desired basic lead carbonate crystals.

In one embodiment of the invention, the aqueous solution of basic lead salt is merely placed in a vessel and the stream of diluted carbon dioxide is then bubbled through it at a rate whereby the pH of the solution is maintained'above about 6.5 and more preferably is maintained within the range of about 6.8 to about 7.3. Accordingly, introduction of the carbon dioxide into the solution is terminated when the pH thereof can no longer be maintained above 6.5, and more preferably is terminated at apI-I which is no lower than about 6.8.

In another and more preferred embodiment of the invention, a stream of the basic lead salt solution is introduced into an agitated body of water while a stream of diluted carbon dioxide is introduced into the water coextensively with the salt solution. This separate, agitated body water thus serves as the reaction medium for the two streams being fed into it, i.e. the carbon dioxide and the salt solution. In these cases also, the pH of the aqueous medium is maintained above about 6.5 during the reaction, and more desirably ismaintained within the range of about 6.8 to about 7.3. The pH of the aqueous medium can be adjusted, if necessary,during addition of the reaction streams by altering the flow rate of either stream to compensate for a change in pH effected by the other.

The reaction period for producing basic lead carbonate in accordance with the present invention is variable, but generally speaking the most desirable results are obtained by very slow reaction of the basic lead salt with carbon dioxide, i.e. a reaction time of at least about six hours. Since, however, the economic practicality of the process is to a large part dependent upon the production rate, an advantage is obtained by limiting the reaction time to a period of time which is within the range of about 6 to about 8 hours.

At the end of the reaction period the crystals of basic lead carbonate are allowed to settle out of the aqueous reaction medium. The resulting precipitate is drained off and centrifuged to produce an aqueous concentrate, or paste, of pigment grade basic lead carbonate which may then be marketed as such. Where preferred, the basic lead carbonate crystals can be placed instead into an organic solvent medium simply by vigorously mixing the aqueous concentrate with the solvent whereby the pigment is transferred, or flushed from the aqueous to the organic phase. In such a case, dioctylphthalate can be employed as the organic solvent to provide a pigment concentrate of about 50 to about 80 weight percent of pigment. in still other cases the aqueous paste of pigment can be demoisturized to provide a dry pigment. Procedures for drying the pigment are well known in the art, i.e. by means of an organic liquid drying agent such as an alcohol, an acetate, a ketone, or an ester.

When the reaction between carbon dioxide and the basic lead salt is carried out in accordance with the procedure described herein, crystal platelets of basic lead carbonate can be produced which are of a highly regular hexagonal shape and which have an average diameter within the range of about to about microns and an average thickness within the range of about 10 to about 80 millimicrons. By optimization and precise regulation of process conditions pigments can be produced wherein the crystal plateletes have an average diameter within the range of about 12 to about 18 microns and an average thickness within the range of about 10 to about 50 millimicrons. Produced accordingly, the pearlescent basic lead carbonate pigment is characterized by a brilliance and luster which equals or excels that of natural pearl essence.

EXAMPLE I Lead monoxide was dissolved in an aqueous solution of acetic acid to provide 20 gallons of basic lead salt solution having a content of lead of 12 weight percent. This solution was allowed to stand overnight, and the clear supematent was then decanted off the residue which had settled out. The resulting purified solution of basic lead salt was then cutback with distilled water to a content of three weight percent.

A stream of carbon dioxide which had been diluted with 50 volume percent of nitrogen was then bubbled slowly into the basic lead salt solution while maintaining the temperature therein at about C. The reaction mixture was maintained under agitation by means of a stirrer, and the feed rate of the carbon dioxide was regulated to maintain the pH of the mixture between about 6.5 and 7.5. When the mixture could no longer be maintained at a pH above 6.5, i.e. at the end of the 6.5 hours, the feeding of carbon dioxide into the reaction mixture was stopped. A sample of the precipitated basic lead carbonate was then removed from the reactor for microscopic examination, and after the precipitate had settled it was drained off and centrifuged to provide a paste which contained less than 5 weight percent of water.

Microscopic examination of the basic lead carbonate produced in this experiment revealed regular hexagonal crystals of basic lead carbonate, the predominate portion of which had a diameter within the range of about 10 to about 20 microns and a thickness within the range of about 10 to about 80 millimicrons.

EXAMPLE ll Lead monoxide was dissolved in an aqueous solution of beta-chloropropionic acid to provide a solution wherein the content of lead was about 2 weight percent. The solution was then filtered to remove trace amounts of objectionable foreign matter. While vigorously stirring the solution and maintaining it at about 65C, carbon dioxide which had been diluted with volume percent of air was bubbled into the solution. The feed rate of the diluted carbon dioxide stream was regulated to maintain the pH of the mixture between about 6.8 and 7.3. When the mixture could not longer be maintained-above about 6.8, i.e. at the end of about 3.5 hours, introduction of the carbon dioxide was stopped. A sample of the percipitate, was recovered for microscopic examination of the pigment. After being allowed to settle, the precipitate was passed to a centrifuge and was recovered therefrom as a paste containing less than 5 weight percent of water.

Microscopic examination of the crystal platelets of basic lead carbonate revealed that a pigment had been produced which wasabout equivalent to that produced in Example I.

EXAMPLE Ill 6.94 kilograms of lead monoxide and 1.772 kilograms of propionic acid were added to a tank which contained 113.7 liters of water at room temperature. This mixture was stirred until dissolution of the lead monoxide was completed. The resulting solution was allowed to stand overnight. The next morning, the clear, supematent solution of basic lead salt was decanted into another tank and was then fed very slowly into still another tank which contained 113.7 liters of water at the start-up of the reaction. Feeding of carbon dioxide into the body of water in the tank was started at the same time. The carbon monoxide feedstream was diluted with 75 volume percent of air. In this case the input rate of the carbon dioxide stream was regulated to maintain the pH of the aqueous reaction medium within the range of about 7.2 to 7.3. The reaction temperature was maintained at about 25C. At the end of 8 hours, the pH of the reaction medium fell to 6.8 and feeding of the carbon dioxide was stopped. A sample of the precipitate was then taken from the reaction tank for examination of the basic lead carbonate crystals. The precipitate was allowed to settle and was then drained out of the tank and centrifuged to produce an aqueous paste of the pigment. This was then mixed with dioctylphthalate and the mixture was vigorously agitated to effect flushing of the pigment into the organic solvent at a solids loading of 60 percent by weight.

Microscopic examination of the basic lead carbonate pigment produced in accordance with this experiment revealed highly regular hexagonal platelets, about 60 percent of which had a diameter within the range of about 10 to about 20 microns and a thickness within the range of about 10 to about 50 millimicrons.

EXAMPLE IV in Example Ill, the carbon dioxide stream was passed through the aqueous reaction medium as bubbles having a diameter of about one-sixteenth of an inch. Example III was repeated but wherein the diluted carbon dioxide was passed through the reaction medium as bubbles having diameters within the range of about one-fourth to about three-fourths of an inch.

Examination of the pigment produced in this experiment indicated a somewhat higher quality, i.e. about 80 percent of the crystal platelets had a diameter within the range of about 12 to about 18 microns and a thickness within the range of about 30 to about 80 millimicrons. V

The present invention has been specifically described with reference to particular materials, conditions, proportions, concentrations, and the like, but it will be understood that still other embodiments will become apparent which are within the spirit and scope of the invention which is defined in the appended claims.

Therefore, what is claimed is:

1. A process for producing a pearlescent basic lead carbonate pigment which comprises introducing a stream of carbon dioxide gas into an aqueous solution of a basic lead salt which contains about 0.5 to about 7 weight percent of the 'salt, said stream being diluted with a 'gas that is substantially nonreactive with the salt, maintaining the temperature of the solution within the range of about C. to about 100C. and maintaining the pH of the solution above about 6.5 during introduction of the diluted stream of carbon dioxide, reacting the basic lead salt with the carbon dioxide over a period of at least about 6 hours, and recovering a pearlescent basic lead carbonate pigment in the form of hexagonal crystalline platelets, the predominate portion of which have an average diameter within the range of about to about 20 microns and a thickness within the range of about 10 to about 80 millimicrons.

2. The process of claim 1 in which said basic lead salt is produced by reacting lead monoxide with an aqueous solution of an organic acid having the general formula:

wherein X is a member from the group consisting of hydrogen, methyl and halogen.

3. The process of claim 2 in which said acid is acetic acid.

4. The process of claim 2 in which said acid is propionic acid.

5. The process of claim 2 in which said acid is betachloropropionic acid.

6. The process of claim 2 in which said acid is isobu tyric acid.

7. The process of claim 2 in which said acid is n-butyric acid.

8. The process of claim 1 in which said basic lead salt is produced by reacting lead monoxide with an aqueous solution of nitric acid.

9. The process of claim 1 in which the stream of carbon dioxide gas is diluted with at least about 25 volume percent of said nonreactive gas.

10. The process of claim 9 in which the nonreactive gas is a member from the group consisting of air, nitrogen, and inert gases.

11. The process of claim 9 in which the nonreactive gas is air.

12. The process of claim 1 in which the stream of carbon dioxide is diluted with from about 50 to about 0] rcent f sa'd nonreactive as.

13. iiig smcess of claim 1 in w rob the diluted stream of carbon dioxide gas is passed through said solution as bubbles having a diameter of at least about one-fourth of an inch.

14. The process of claim 1 in which a stream of said aqueous solution of basic lead salt is introduced into a body of water while said stream of carbon dioxide gas is passed into said body of water coextensively with said salt solution, said body of water serving as the reaction medium for producing the basic lead carbonate.

15. The process of claim 14 in which said streams of basic lead salt solution and carbon dioxide are coextensively introduced into said body of water for a period of time within the range of about 6 to about 8 hours.

16. The process of claim 1 in which the pH of the solution is maintained between about 6.8 and about 7.3 during introduction of the carbon dioxide. 

2. The process of claim 1 in which said basic lead salt is produced by reacting lead monoxide with an aqueous solution of an organic acid having the general formula: wherein X is a member from the group consisting of hydrogen, methyl and halogen.
 3. The process of claim 2 in which said acid is acetic acid.
 4. The process of claim 2 in which said acid is propionic acid.
 5. The process of claim 2 in which said acid is betachloropropionic acid.
 6. The process of claim 2 in which said acid is isobutyric acid.
 7. The process of claim 2 in which said acid is n-butyric acid.
 8. The process of claim 1 in which said basic lead salt is produced by reacting lead monoxide with an aqueous solution of nitric acid.
 9. The process of claim 1 in which the stream of carbon dioxide gas is diluted with at least about 25 volume percent of said nonreactive gas.
 10. The process of claim 9 in which the nonreactive gas is a member from the group consisting of air, nitrogen, and inert gases.
 11. The process of claim 9 in which the nonreactive gas is air.
 12. The process of claim 1 in which the stream of carbon dioxide is diluted with from about 50 to about 80 volume percent of said nonreactive gas.
 13. The process of claim 1 in which the diluted stream of carbon dioxide gas is passed through said solution as bubbles having a diameter of at least about one-fourth of an inch.
 14. The process of claim 1 in which a stream of said aqueous solution of basic lead salt is introduced into a body of water while said stream of carbon dioxide gas is passed into said body of water coextensively with said salt solution, said body of water serving as the reaction medium for producing the basic lead carbonate.
 15. The process of claim 14 in which said streams of basic lead salt solution and carbon dioxide are coextensively introduced into said body of water for a period of time within the range of about 6 to about 8 hours.
 16. The process of claim 1 in which the pH of the solution is maintained between about 6.8 and about 7.3 during introduction of the carbon dioxide. 